In recent years, due to the progress of digital image processing techniques, digital type image formation has become a mainstream even in image forming methods utilizing electrophotography in which electrostatic latent images are developed.
The digital type image forming method is based on the visualization of latent images comprised of minute dots such as 1,200 dpi (dots per inch; the number of dots per 2.54 cm). Therefore, high image quality techniques are sought which faithfully reproduces such minute dot images. In order to realize such high image quality, electrostatic latent image developing toner has been subjected to a decrease in toner particle diameter as well as a decrease in the range of the particle size distribution and an increase in particle shape uniformity.
In image forming methods which develop electrostatic latent images, heretofore, so-called pulverized toner has mainly been employed, which is prepared in such a manner that binder resins and pigments are mix-kneaded, subsequently pulverized, and the resulting toner powder is classified. However, such pulverized toner exhibits limit in the decrease of toner particle diameter as well as the decrease in the particle size distribution range and uniformity of particle shape. As a result, it has been difficult to produce the desired increasingly high quality images as long as such pulverized toner is employed.
In recent years, attention has given to so-called polymerization method toner which is prepared employing either a suspension polymerization method or an emulsion polymerization method as a means to achieve a decrease in the diameter of toner particles as well as the uniformity of the particle size distribution and the shape. The polymerization method toner is prepared in such a manner that monomers as a raw material are uniformly dispersed in a water based medium, followed by polymerization and then employed. Various methods to prepare the polymerization method toner are known. Of these, a method, which receives marked attention, is that resinous particles, prepared by the suspension polymerization method or the emulsion polymerization method, are coalesced (salting out and fusion) with colorant particles. This method makes it easy to prepare toner particles having a smaller diameter and a more uniform particle size distribution. As a result, its practical application has been investigated, however, the aspect of its production engineering is still during the stage of development.
On the other hand, widely accepted as methods to fix toner images formed on image forming supports, such as paper, is heating roller fixing which achieves fixing by transporting the image forming support, on which said toner images are formed, between a heating roller and a pressure roller.
However, when the heating roller fixing is applied to the polymerization method toner comprising small diameter toner particles and exhibiting uniform particle size distribution as well as uniform shape, problems occur in which insufficient fixing results, due to the fact that toner particles are not sufficiently deformed so as to fit into the unevenness of paper as well as the unevenness of the surface of the heating roller.